According to the ITTF Handbook, the rule about diagonal serves only applies to doubles:
As the ball is falling the server shall strike it so that it touches first his or her court and then, touches directly the receiver's court; in doubles, the ball shall touch successively the right half court of server and receiver.
Consider the following example, where it is assumed that all ITTF rules are followed:
I had served several balls/points diagonally, but in the next serve, while maintaining the same stance and position as I had in prior serves, I serve the ball to the other side of the table.
Would this be considered bad sportsmanship?
Do professionals do this in matches?
It would be great to see e.g. videos of this happening to get an idea of how it happens in official matches, if at all.